The invention relates to a method for the treatment of fly ash in a waste incinerating plant with liquid slag discharge, comprising a revolving cylindrical furnace provided with charging equipment for solid, pasty and liquid waste materials as well as sludge and which at its end has a melting slag bath, a secondary combustion chamber, and a flue gas dust separator, whereby the fly ash separated from the flue gases by means of the latter is discharged together with the slag by means of the incinerator slag discharge system.
The invention also relates to a plant for performing this method.
In order to avoid atmospheric pollution, particular importance is attached to removing dust from flue gases when burning waste materials which are rich in ballast. Whereas, when removing the dust from flue gases by means of electrostatic filters, cyclones or cloth and/or tube filters the separated dust is discharged in the dry state. When using a gas scrubber this leads to a wet dust discharge in the form of sludge. The present invention is in particular based on so-called dry flue gas dust separation.
Herein the dust separated by a flue gas dust separator is called "fly ash", whereas the dust still carried by the flue gases downstream of the gas dust separator and which finally pass through the chimney into the atmosphere is called "flue dust".
If the fly ash is discharged dry from a waste incinerator, it is in known manner stored under field conditions, either separately or together with the slag.
As is known, the composition of fly ash differs considerably from the composition of slag from municipal refuse incinerators and the composition of slag from industrial refuse incinerators which has been melted in a revolving cylindrical furnace. The following table of approximate compositions illustrates these differences:
______________________________________ Slag from: communal industrial refuse refuse Part Fly ash incinerators incinerators ______________________________________ Combustible (% by weight) 5-10 2-6 0-2 Ash (% by weight) 90-95 94-98 98-100 Soluble part (% by weight) 20-50 approx. 1 1-3 ______________________________________
The proportion of easily volatized heavy metals, such as e.g. zinc, manganese, or lead is many times higher in the fly ash than in the slag, while the melting point of the fly ash is generally lower than that of the slag. The water absorption capacity of molten slag is approx. 1 to 2% by weight.
In a known method of fly ash removal, the slag, together with the admixed fly ash, is dumped under field conditions. However, the fly ash, which contains in addition to its unburned organic constituents also a large proportion of water-soluble substances, as well as easily volatized heavy metals, such as e.g. tin, manganese, lead, etc. (cf. above table) can cause a contamination of the soil and the ground water. In addition, the transportation of the slag to a dump and the storage thereof in the latter can cause serious dust pollution of the surrounding atmosphere.